This application is based on Korean Patent Application No""s. 98-5019, 98-13126, 98-30385, and 99-856 the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a display apparatus, and more particularly, to a video preamplifier circuit for composing an on-screen display signal with a video signal, and for amplifying the result on the display apparatus.
2. Description of Related Arts
In a cathode ray tube (CRT) monitor or a liquid crystal display (LCD) used for a computer, for example, it is often necessary to display information such as characters on a video picture via an on-screen display (OSD). Examples of such OSD information include a user-controlled contrast or convergence level bar and the title of the item currently being adjusted. The mixing of such OSD information with the underlying video signal is performed in a video preamplifier integrated circuit (IC).
When the OSD characters are to be displayed, it is important to emphasize the characters so that they can be easily distinguished from the video picture. Shadowing techniques for shadowing certain edges of the characters as shown in FIG. 30A, or bordering techniques for emphasizing the outline of the characters as shown in FIG. 30B, may be employed for this purpose.
In another character emphasis method, an OSD window may be formed on the screen with only the characters being displayed in the window, while intercepting, or overwriting, the video signal in the window area. FIG. 25 and FIG. 30C are exemplary video images on which the OSD characters are displayed on the OSD window. In such an OSD window area, a background window 501 is formed in one color and the OSD characters 503 are displayed on the background screen. Typically, the OSD raster for forming the background portion is fixed in black or cyane.
Meanwhile, if the video signal is completely intercepted in the window area when the OSD window is formed on the screen, the quality of an image is considerably deteriorated because the original video image is completely overwritten by the OSD background window. Accordingly, in recent display systems, a half tone video image is displayed on the OSD background window by reducing the gain of the video signal by half as shown in FIG. 30D, instead of complete replacement of the video signal in the window area. In such a case, the OSD characters are displayed on the half-tone image.
However, a standard video preamplifier integrated circuit (IC) capable of displaying the half tone image in the OSD window generally does not support an operation mode of selectively filling the OSD background window by either the half tone image or an OSD background raster other than white. Thus, in a conventional OSD window generating system, in the video preamplifier IC, the OSD background window may be filled by an OSD background raster of a specific color or a half tone video image with a white background raster. Furthermore, in an IC which fills the OSD background portion with an OSD raster of a specific color, the color of the OSD background raster is generally fixed and cannot be changed.
Therefore, consumers of the video preamplifier IC, such as a monitor manufacturer, are limited to a single operation mode, and are forced to purchase another IC when the method of displaying the OSD window is to be changed according to the use of the monitor.
In order to solve the above problems, one object of the present invention is to provide an on-screen display system for displaying OSD information on an OSD background which is selectively filled with either a half tone video image or a background raster of a user-selectable color, or which is filled by superimposing the half tone video image and the half tone background raster selected by the user.
Another object of the present invention is to provide an on-screen display system for displaying a plurality of OSD windows and filling each of the plurality of OSD windows separately by filling with either a half tone video image or a background raster of a user-selectable color, or by superimposing the half tone video image and the half tone background raster selected by the user.
In the on-screen display system according to the present invention, to achieve one of the above objects, three mixing and amplifying circuits are provided respectively for each of red, green, and blue (R/G/B) channels. Each of the three mixing and amplifying circuits receives a video signal and an OSD signal of a respective channel, mixes the video signal and the OSD signal according to a video selection signal and an OSD selection signal, amplifies a mixed signal, and outputs an amplified image signal to a display. A selection signal generating circuit receives OSD signals for R/G/B channels, a video/OSD switching signal, a half tone signal, and OSD control signals, and generates and outputs the OSD selection signal and the video selection signal. When the video/OSD switching signal instructs an OSD output operation and the half tone signal instructs a half tone output operation, the selection signal generating circuit outputs the video selection signal and the OSD selection signal such that a half tone OSD background raster is displayed with a half tone video image in a predetermined OSD window.
According to an aspect of the present invention, the selection signal generating circuit is implemented by using logic gates. In the selection signal generating circuit, when the video/OSD switching signal instructs an OSD output operation, the half tone signal instructs a half tone operation, and the OSD signals have a predetermined logic combination, then a half tone display signal generator generates an active half tone display signal. An inverter inverts the video/OSD switching signal to output a video mode signal which is activated when only the video signal is to be output. An attenuator attenuates the half tone display signal to a fraction of, for example half, its original value. An adder adds the video mode signal to the output of the attenuator to output the addition result as the video selection signal and the OSD selection signal.
According to another aspect of the present invention, the selection signal generating circuit is implemented by using transistors and passive elements. In the selection signal generating circuit, a video/OSD switching unit receives the video/OSD switching signal and generates at least one switching signal according to the video/OSD switching signal. An OSD raster selector adjusts the swing ranges of the R/G/B OSD signals, determines whether each of the level-adjusted R/G/B OSD signals is at a first or second logic level, and outputs a raster selection signal when the R/G/B OSD signals corresponds to a predetermined combination of logic levels which is determined according to the OSD control signals. A half tone signal generator receives the switching signal and the raster selection signal from the video/OSD switching unit and the OSD raster selector, respectively, and generates the OSD selection signal and the video selection signal.
To achieve another one of the above objects, the on-screen display system of the present invention forms a first predetermined number of OSD windows in a displayed image and displays OSD information and video information in half tone in a second predetermined number of OSD windows among the first predetermined number of OSD windows. Three mixing and amplifying circuits are provided respectively for each of R/G/B channels. Each of the three mixing and amplifying circuits receives a video signal and an OSD signal of a respective channel, mixes the video signal and the OSD signal according to a video selection signal and an OSD selection signal, amplifies a mixed signal, and outputs an amplified image signal to a display. A selection signal generating circuit receives OSD signals for R/G/B channels, a video/OSD switching signal, a half tone signal, and OSD control signals for the second predetermined number of OSD windows, and generates and outputs the OSD selection signal and the video selection signal. When the video/OSD switching signal instructs an OSD output operation and the half tone signal instructs a half tone output operation, the selection signal generating circuit outputs the video selection signal and the OSD selection signal such that a half tone OSD background raster is displayed with a half tone video image in the second predetermined number of OSD windows.
In a preferred embodiment, the video selection signal and the OSD selection signal are identical and a single video/OSD selection signal is used. Each of the mixing and amplifying circuits amplifies only the video signal to output an amplified video signal when the video/OSD selection signal is at a first logic level, and only the OSD signal to output an amplified OSD signal when the video/OSD selection signal is at a second logic level. Each of the mixing and amplifying circuits mixes the video signal and the OSD signal to output an amplified and mixed signal when the video/OSD selection signal is at a medium logic level.
According to another aspect of the present invention, the selection signal generating circuit is implemented by using logic gates. In the on-screen display system, a gating signal generating unit generates a gating signal which is activated when the video/OSD switching signal instructs an OSD output operation and the half tone signal instructs an half tone operation. A first OSD determining unit receives the gating signal, OSD signals for the R/G/B channels, and the OSD control signals for a first window, and generates a first window discriminating signal which is activated only when the OSD signals have a first predetermined combination corresponding to the OSD control signals for the first window and the gating signal is activated. A second OSD determining unit receives the gating signal, OSD signals for the R/G/B channels, and the OSD control signals for a second window, and generates a second window discriminating signal which is activated only when the OSD signals have a second predetermined combination corresponding to the OSD control signals for the second window and the gating signal is activated. An OR gate performs an OR operation on the first window discriminating signal and the second window discriminating signal. An attenuator receives an output of the OR gate and attenuates the output of the OR gate to a half. An inverter receives and inverts the video/OSD switching signal and generates a video mode signal activated when only the video signal is to be output. An adder adds the output of the attenuator to the video mode signal and outputs the addition result as the video/OSD selection signal.
According to another aspect of the present invention, the selection signal generating circuit is implemented by using transistors and passive elements. In the selection signal generating circuit, an OSD raster selector receives the OSD signals for R/G/B channels, and the OSD control signals for the first and second windows, and generates at least one raster selection signal which is enabled when the OSD signals have a logic combination corresponding the OSD control signals for the first window or the OSD control signals for the second window. A half tone signal generator receives the raster selection signal, the video/OSD switching signal and the half tone signal, and generates the video/OSD selection signal of medium level when the video/OSD switching signal instructs the OSD output operation, the half tone signal instructs the half tone output operation, and at least one of the raster selection signals is enabled.